what size outboard for 18' bowrider

[malcsmith]

Hi,

I would like to get a small outboard as an auxilliary back-up in case my engine fails. What size do you think I would need to get me home. My boat is a wellcraft 180 with a 3.0l mercruiser engine in it.

Cheers,

Malc

 

[dpb]

I think a 5hp would do as a 9.9hp (I think thats the next size up) may be a little large physically.
Some say that a way of drawing the fuel from the main tank is a good idea.
Main issue would be can you fix it so that it is low enough in the water to work with the prop in the water all the time, and how will the boat be steered.
Personally I would consider how you use the boat.
If you are off in remote areas then there is a clear point in having an aux motor set up that works.
But if you are in an area like Poole Harbour and only go out on good weather days then you are not going to be alone out there and having a good means of communication and a suitable tow rope is arguably a more practical bet.
Will be interesting to hear what others view is.

 

[Hobnob]

Will be interesting to hear what others view is.

1hp per metre of boat, plus a suitable low-pitched prop, e.g. saildrive type.

And a separate fuel supply for enhanced reliability.

 

[dpb]

And a separate fuel supply for enhanced reliability.

But then you have to keep that supply fresh and are limited in range by the size of the separate supply tank.......then again arn't most petrol breakdowns at sea due to running out of fuel!!??!

 

[grumpy_o_g]

If you want to punch against the ebb to get in to Christchurch then look at the 10 hp end of the spectrum. You'll need to be able to do at least 5 knots, preferably 6 or 7. If you can wait for the worst of the ebb to pass then you'll only need a small outboard. A few hp with the right prop will be fine - look for something designed to act as an auxiliary for a sailing boats rather than something designed for lightweight tenders. Assuming you won't be out in nasty weather in a bow rider an decent anchoring set up might be a good idea so you can wait for something nearer slack water.

This might be a useful thread to give you some idea of small outboards and how they drive that size boat http://www.ybw.com/forums/showthread.php?t=319441. I don't think he's nasty to MoBo'ers in this one

 

[malcsmith]

Thanks for your help so far!

Does anyone know whether I would need a short shaft or a long shaft?

The boat has a platform for mounting the outboard onto, which lowers down when required. I'll try and get some photos.

 

[longjohnsilver]

Thanks for your help so far!

Does anyone know whether I would need a short shaft or a long shaft?

The boat has a platform for mounting the outboard onto, which lowers down when required. I'll try and get some photos.

I'd go for a long shaft, better to have too much in the water rather than not enough and suffer constant cavitation as the boat pitches.

 

[PaulGooch]

Worth noting that some 4/5/6hp outboards are the same engine, different carb. For instance, Mercury, Mariner and Tohatsu. In fact, all three brands and all three HP's are the same engine, different livery for each "manufacturer" and different carb for each HP.

So, a 6 hp will be the same size and weight as a 4/5 hp.

 

[Sheppy]

Bear in mind that some only have an integral tank, some only have an external tank feed while some have both. The Suzuki 4/5/6 for example is the same engine therefore the same weight but the 4 only has an integral tank.

 

[PaulGooch]

Bear in mind that some only have an integral tank, some only have an external tank feed while some have both. The Suzuki 4/5/6 for example is the same engine therefore the same weight but the 4 only has an integral tank.

Yeah, and it only holds about 1ltr of fuel. The 6hp Mercury has a nice external tank (on integral one).

 

[Sheppy]

Yeah, and it only holds about 1ltr of fuel. The 6hp Mercury has a nice external tank (on integral one).

Most do only have a small tank but the Suz is supposed to have a 1.4L or so the chap at the show told me. He reckons it runs for 1.5 hours at near full rpm on a single tank. I thought that was better than having to have a separate tank, leads etc..
I'm looking at one as my aux but they're soooooo expensive!

 

[PaulGooch]

Most do only have a small tank but the Suz is supposed to have a 1.4L or so the chap at the show told me. He reckons it runs for 1.5 hours at near full rpm on a single tank. I thought that was better than having to have a separate tank, leads etc..
I'm looking at one as my aux but they're soooooo expensive!

Had a brand new one, didn't seem to last too long on my tender. I traded it in at 6 months old for a 6HP Merc with the separate tank, it wasn't really man enough in a strong head tide, for my liking. I preferred the separate tank, but it obviously doesn't suit everyone. Pretty sure the 6hp Mariner (same engine different stickers) had an integral tank, possibly with the connector for the external tank, if memory serves.

 

[ontheplane]

Many people buy the 6hp type outboard, but then leave the standard prop on - DO NOT DO THIS!

The standard prop on this size outboard is set up that at max rpm it will push a tiny tender up to max hull speed or even pop it up on the plane. You are not trying to do this when using as an aux engine.

Make sure you either buy a "sailpower" type outboard, or at the very least fit a saildrive propellor - which will have a larger blade area, and a much lower pitch allowing the engine to run up to it's max rpm and provide less speed but much more thrust (which is what you need).

Sailpower engines are the best however, they are geared differently, they have a larger diameter prop, a higher thrust for a given hp and often have a 12v output to charge the boat's batteries (useful if your need for the aux is a flat battery).

And to go to a point made earlier - yes having a feed from the boats tank is a good idea, but make sure you have a small aux tank as well in case you need the auxiliary engine because you've run out of fuel!

On keeping the aux outboard's fuel fresh, what I always used to do was empty the small tank into the main vessels main tank, and refill it each time we went to the boat - that way the fuel was always fresh.

 

[malcsmith]

Thanks for that - very helpful!

 

[lovezoo]

Also bear in mind a secondhand 2 stroke can't just be run from the main fuel tank on the boat, as it needs 2 stroke oil in the fuel.

As already said though, depending where you are planning to go on your boat and how many other boats there are likely to be around, a strong tow rope might be a better fool proof alternative to an spare outboard that never gets used.